With the development of the future service, next generation wireless communication systems, such as 3GPP (third Generation Partnership Project) LTE (long term evolution) and beyond system, IMT-A (International Mobile Telecommunications-Advanced) system etc., are introduced to satisfy high speed, large capacity, and a high QoS (Quality of Service) for billions of subscribers. In this regard, efforts have been made to realize network-controlled D2D communications for reducing the load on the cellular communication network. Examples of such D2D communications include direct communications among a cluster of proximity devices, and autonomous D2D communications in a cellular network. In such network-controlled D2D communications, devices such as user equipments (UEs) or mobile terminals directly communicate with each other, instead of conveying data from one device to the other via the cellular network (in particular via an access node or base station thereof), where primary control and configurations, such as channel/bearer configurations, are carried out by the cellular network. Security protection may be an issue for the network-controlled D2D communications, for example, because malicious users may be able to eavesdrop on the D2D communication if no strong security protection between peer UEs conducting a direct D2D communication is used. However, currently the security related procedures have not been fully specified for network-controlled D2D communications, especially for a scenario that one of the peer UEs in the D2D communication stays in an idle mode.
In view of this, it would be advantageous to provide a way to allow for efficiently securing D2D communications, especially when one of the D2D UEs is in an idle mode.